The patent on the landmark HIV/AIDS drug Combivir will expire on May 15, but there's already a far less expensive generic version of the drug on the market. The FDA approved Combivir (lamivudine/zidovudine) in 1997, greatly changing the treatment of HIV/AIDS. It was the first drug to combine two widely-used antiretroviral medications into a single pill, making it easier for patients to fight the disease.

Davey Smith, an infectious diseases researcher at the UC San Diego School of Medicine, said, "The generic form should work just as well as the branded form. Most switches to generics are made at the pharmacy level, not at the doctor level, and the switch to generics is mostly determined by the insurance provider."

The generic version is marketed by Teva Pharmaceutical and generated $315 million in the U.S. market last year. Drug prices vary. But it would not be uncommon for the brand version of Combivir to sell for $562, and for the generic to cost about $170 less. Patients should consult their pharmacist and insurer.